Highly diastereoselective inverse electron demand (IED) Diels-Alder reaction mediated by chiral salen-AlCl complex: The first, target-oriented synthesis of pyranoquinolines as potential antibacterial agents

Article abstract of DOI:10.1016/j.bmcl.2004.02.057

The first, target-oriented synthesis of pyranoquinolines as potential antibacterial agents by inverse electron demand (IED) Diels-Alder reaction using chiral salen-AlCl complex has been accomplished, the reactions proceed with moderate yields, and a very

Full text of DOI:10.1016/j.bmcl.2004.02.057

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2035–2040
Highly diastereoselective inverse electron demand (IED)
Diels–Alder reaction mediated by chiral salen–AlCl complex:
the first, target-oriented synthesis of pyranoquinolines
as potential antibacterial agents
Chinnian J. Magesh, Sarasu V. Makesh and Paramasivan T. Perumal*
Organic Chemistry Division, Central Leather Research Institute, Adyar, Chennai 600 020, India
Received 31 December 2003; revised 13 February 2004; accepted 13 February 2004
Abstract—The first, target-oriented synthesis of pyranoquinolines as potential antibacterial agents by inverse electron demand
(IED) Diels–Alder reaction using chiral salen–AlCl complex has been accomplished, the reactions proceed with moderate yields, and
a very high degree of diastereoselectivity (>90%). The diastereoselectivity-enhanced pyranoquinolines exhibit potential bactericidal
activity against seven strains of pathogenic gram-negative bacteria.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
In our quest to synthesize new lead compounds with
potential antibacterial activity the products of the IED-
DA reaction between 3,4-dihydro-2H-pyran and 2-aza-
diene were screened for their antibacterial activity
(Scheme 1). The exo-diastereomer 7 exhibited potential
bactericidal and bacteriolytic activity against seven
strains of pathogenic gram-negative bacteria with pro-
ven clinical syndromes associated with human beings
while the endo-diastereomer 6, was not only inactive but
also brought about the competitive inhibition of the
active diastereomer 7 by binding either to the receptor
sites on the bacteria or to some component of the
effector mechanism, thereby preventing the active dia-
stereomer 7 from having an effect. Hence to, synthesis
the biologically active target, avoid interference from the
inactive isomer, to overcome the diastereomer 6 medi-
ated competitive inhibition of the active isomer 7 and to
circumvent the time consuming and laborious separa-
tion of diastereomers, we explore the possibility of using
chiral salen–AlCl complexes for a target-oriented syn-
thesis of the active diastereomer by enhancement of
diastereoselectivity for IED Diels–Alder reaction.
The pyranoquinoline moiety is present in many alka-
loids such as flindersine, oricine and verprisine1 and
derivatives of these alkaloids possess a wide range of
biological activities such as psychotropic,2 antialler-
genic3 and antiinflammatory4 activities. The furoquino-
line skeleton is present in many alkaloids like
skimmianine and balfouridine.5;6 For synthesis of
quinoline products various methods are available, but
reports on the synthesis of diastereoselective quinoline
derivatives are limited.7 Over the years, conventional
antibiotics have been used so extensively that many of
the bacteria that infect us have grown resistant to them.
Moreover, no new classes of antibiotics have been
developed in over 30years and bacteria are learning how
to outsmart existing drugs and there appear to be few, if
any, new classes of drugs currently in clinical develop-
ment. The need for research directed towards develop-
ment of new lead compounds with potential
antibacterial activity has never been greater.
Keywords: Inverse electron demand (IED) Diels–Alder reaction; Chiral
salen–AlCl complex; Diastereomeric excess; Pyranoquinolines; Bacte-
ricidal and bacteriolytic agents.
2. Chemistry
Salen H2 ligands occupy five coordination sites on the
central aluminium atom but presumably leave the sixth
* Corresponding author. Tel.: +91-44-24913289; fax: +91-44-2491-
1589; e-mail: ptperumal@hotmail.com
0960-894X/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.02.057

2036
C. J. Magesh et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2035–2040
Scheme 1.
site open for substrate coordination. Thus these salen–
AlCl complexes behave as coordinatively unsaturated
species in the presence of a more basic substrate, such as
the 2-azadiene. Here the reversible complexation of
chiral salen–AlCl to the 2-azadiene would make the
2-azadiene more reactive in a concerted Diels–Alder
process, because this will decrease the energy gap be-
tween the LUMO (2-azadiene) and HOMO (dieno-
phile). The chiral salen–AlCl complexes were prepared
by combining the appropriate salen H2 ligands with
R2AlCl.8 The electron-poor diene, benzylidene aniline
was prepared and recrystallized as per literature.9
On examining the role of the solvent towards improving
the diastereoselectivity of this reaction (Scheme 1), we
found that with pure toluene there was no reaction. In
dichloromethane medium alone, the products were
obtained in poor yield. On switching to a more polar
solvent like methanol, the amount of conversion im-
proved considerably but the diastereomeric excess were
still considerably low, possibly as a result of a high
degree of solvation leading to the delinking of the ligand
from the metal. However of all the solventÕs examined,
acetonitrile proved to be the most effective with a very
high degree of diastereoselectivity (Table 1).
H
H
H
H
N
N
N
N
Al
Cl
Al
Cl
_
_
MeO
O
O
OMe
t
Bu
O
O
t
Bu
_
_
_
_
t
t
Bu
Bu
t
Bu
t
Bu
1a
1b
Ph
Ph
Ph
Ph
H
H
H
H
N
N
N
N
Al
Cl
Al
Cl
_
_
O
O
O
O
t Bu
OMe
MeO
t
Bu
_
_
_
_
t
Bu
t
t
Bu
Bu
t
Bu
2b
2a
CH3
H
CH3
H
N
N
N
N
Al
Cl
Al
Cl
_
_
O
O
MeO
OMe
t
O
O
Bu
t
Bu
_
_
_
_
t
Bu
t
t
Bu
Bu
t
Bu
3b
3a
Table 1. Effect of solvent on the IED Diels–Alder reaction
S. no.
Solvent
Salen–AlCl
Time (h)
Conversion (%)
endo:exo Ratio (6:7)
% De
1
2
3
4
5
Toluene
1a
1a
1a
1a
1a
––
6
––
17
––
––
88
90
Toluene:DCM (1:1)
DCM
Methanol
06:94
05:95
48
6
23
9026:74
63
8
Acetonitrile
6
03:97
94

C. J. Magesh et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2035–2040
2037
Table 2. Effect of substituents on the salen–AlCl complex Table 3. Effect of molecular sieves on the IED Diels–Alder reaction
ꢀ
Entry Salen– 4 A mol Time Conversion endo:exo % De
Entry Salen–AlCl Time
(h)
Conversion endo:exo
(%)
% De
Ratio
(6:7)
AlCl
sieves
(mol %)
(h)
(%)
Ratio
(9:10)
1
2
3
4
5
1b
2a
2b
3a
3b
7
12
14
6
57
45
43
55
47
02:98
01:99
00:100
05:95
04:96
96
98
1
2
3
1a
1a
1a
07
107
207
43
2:5908
47
5:95
96
0
0
90
94
0
100
90
3:97
6
92
Generally imines derived from phenylglyoxal are highly
hygroscopic, unstable at high temperatures, difficult to
purify by distillation or column chromatography and
lack efficiency.11 Henceforth we attempted a three
component coupling reaction by adding phenylglyoxal
monohydrate, aniline and 2,3-dihydro-2H-furan in
the presence of salen–AlCl complex 2b (Scheme 3). The
reaction proceeded with 40% conversion after 7 h in the
endo:exo ratio of 10:90.
The stereochemistry of the products was assigned based
on the scalar-coupling constant between H-4a and H-5.
In the isomer 6, the coupling constant Jð4a-5Þ ¼ 2:2 Hz
is significantly smaller and typical for a gauche confor-
mation. This orientation is present in both conformers
of the configuration having cis orientation of the pyran
ring and phenyl group. In the isomer 7, the
Jð4a-5Þ ¼ 11:4 Hz is indicative of the anti reciprocal
orientation of protons H-5 and H-4a. This orientation is
only possible when the pyran ring and phenyl ring are
on opposite sides of the quinoline ring of 7. On the basis
of reactivity of the salen–AlCl complex 1a in CH3CN,
several further derivatives of salen–AlCl complexes were
examined to probe the effect of substituents on the salen
backbone, in enhancing the diastereoselectivity of the
reaction. Even more effective catalysis was observed
with 2b resulting in 43% conversion after 14 h at room
temperature with 100% de of the biologically active
diastereomer 7. The effect of substituents on the salen–
AlCl complex are summarized in Table 2.
Although the mechanistic details of the present reaction
are still under investigation, a hypothetical mechanism
for the salen–AlCl catalyzed IED-DA reaction is pro-
posed in Scheme 4. The chloride atom on salen–AlCl
complex can easily be displaced by a more basic sub-
strate, such as the 2-azadiene leading to the formation of
an intermediate hexacoordinate octahedral Schiff base
aluminium complex. Thus the catalytically active species
in Schiff base–AlCl complex may be an aluminium cat-
ion.12 Thus, a four centred transition state, with two
open coordination sites occupied by 2-azadiene and
cationic aluminium centre is proposed for the IED-DA
reaction.
It is well known that the addition of molecular sieves to
the reaction mixture sometimes increases the enantio-
meric excesses and yield of the products obtained.10
Therefore to observe the effect of molecular sieves on the
diastereoselectivity of the IED-DA reaction catalyzed by
salen–AlCl complex, we added various mol% of the
molecular sieves to the reaction mixture of 2,3-di-
hydrofuran and 2-azadiene, and observed a marginal
increase in the diastereomeric excess. The results are
summarized in Table 3 (Scheme 2).
3. Antibacterial activity
The cell wall of gram-negative bacteria is a phospholipid
bilayer made up of negatively charged phosphate
groups. Negative charges on the phospholipids are
essential for lysosomal phospholipases to act on
H
H
O
O
O
_
Salen AlCl
+
+
4 A Mol. Sieves
CH3CN, r.t.
N
4
Ph
N
H
N
H
H
9
H
Ph
Ph
8
10
Scheme 2.
Scheme 3.

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C. J. Magesh et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2035–2040
O
O
+
+
O
O
N
N
Al
Cl
Ph
Ph
_
+
O
+
_
O
O
O
Al
Al
Cl
Cl
Azadiene
(Hexa coordinate Octahedral Schiff Base Al Complex)
N
_
_
2
Azadiene
2
Ph
=
_
2
Azadiene
H
o
H
o
H
H
+
O
H
+
O
N
N
Ph
Ph
H
_
_
+
+
O
O
O
O
Al
Al
Cl
Cl
2
Azadiene
_
_
2
Azadiene
_
N
H
H
Cl
O
O
Ph
+
Al
+
N
HN
H
_
2
H
Azadiene
H
Ph
Ph
(Hexa coordinate Octahedral
Schiff Base Al Complex)
6
7
Scheme 4.
phospholipid bilayers. We anticipated that the –NH-
group and the –O-group on the pyranoquinolie moiety
may bind with the negatively charged phosphate group
on phospholipids. This in turn will cause the inhibition
of the activities of lysosomal phospholipases because of
the neutralization of the negative charges of phospho-
lipid bilayer, leading to potential antibacterial activity.
Moreover the outer cell wall and the plasma membrane
of the gram-negative bacteria are permeable to small
molecules. Henceforth the diffusion of the pyranoquino-
line molecule across cell membrane and thereby binding
with intracellular targets may bring about potential
antibacterial activity. However when a 50:50 mixture of
pyranoqinoline diastereomers 6 and 7 were screened for
their bactericidal activity, no activity was found. Out of
the two-pyranoqinoline diastereomers 6 and 7, only the
exo-diastereomer 7 has the desired bactericidal and
bacteriolytic activity while the endo-diastereomer 6 is
inactive even at very high concentrations. The contam-
ination of the active diastereomer 7 even with low con-
centration of 6 significantly lowers its activity. The
biologically inactive isomer 6, binds either to the
Table 4. In vitro screening of pyranoquinoline diastereomers 6 and 7 for bactericidal activity
S. no. Pathogens
Clinical syndrome associa- Concentration
Bactericidal activity
Bacteria
6
6 + 7
(50:50%)
6 + 7
(25:75%)
7
(100%)
ted with human beings
(mg/mL)
(100%)
1
Vibrio
Gastroenteritis, rare wound
infections
2
––
––
––
+
parahaemolyticus
Vibrio alginolyticus
Vibrio anguillarum
Vibrio vulnificus
Vibrio fluvialis
2
3
4
5
6
7
Wound infections
––
2
2
2
2
2
2
––
––
––
––
––
––
––
––
––
––
––
––
––
––
––
––
––
––
+
+
+
+
+
+
Primary septicemia
Gastroenteritis
Gastroenteritis
Vibrio mimicus
Pseudomonas species
Urinary tract infections,
chronic lung infections,
endocarditis and dermatitis
Urinary tract infections,
neonatal meningitis
8
E. coli
2
––
––
––
+
(+): Strong bactericidal activity was observed.
(––): Bactericidal activity was not observed.

C. J. Magesh et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2035–2040
2039
Table 5. In vitro screening of the pyranoquinoline diastereomer 7 for bacteriolytic activity (de ¼ 100%)
S. no.
Bacteria
Bacterial
Bacteriolytic activity
suspension in
optical
density (OD)
Test 1
Test 2
Initial
OD
Final OD Activity
OD
BA
Initial
OD
Final OD Activity
OD
BA
1
2
3
4
5
6
7
8
Vibrio parahaemolyticus 0.8
0.833
0.833
0.833
0.833
0.833
0.833
0.833
0.833
0.718
0.761
0.753
0.720
0.814
0.757
0.769
0.788
0.115
0.072
0.080
0.113
0.019
0.076
0.064
0.045
+++
++
++
+++
––
0.833
0.833
0.833
0.833
0.833
0.833
0.833
0.833
0.712
0.756
0.767
0.731
0.812
0.748
0.764
0.791
0.121
0.077
0.066
0.102
0.021
0.085
0.069
0.042
+++
++
++
+++
––
Vibrio alginolyticus
Vibrio anguillarum
Vibrio vulnificus
Vibrio fluvialis
Vibrio mimicus
Pseudomonas species
E. coli
0.8
0.8
0.8
0.8
0.8
0.8
0.8
++
++
+
++
++
+
(+): Mild bacteriolytic activity, (++): moderate bacteriolytic activity, (+++): strong bacteriolytic activity.
(––): bacteriolytic activity was not observed, BA ¼ bacteriolytic activity.
receptor sites on the bacteria or to some component of
the effector mechanism, in order to prevent the active
isomer 7 from having an effect. Henceforth the target-
oriented synthesis13 of pyranoquinoline 7 with a dia-
stereomeric excess of 100% offers the advantages of
enhanced activity, smaller doses, increased specificity
and completely rules out the competitive inhibition of
the active diastereomer 7, ultimately reaching the max-
imal bactericidal14 and bacteriolytic effect.15 The active
pyranoquinoline diastereomer 7 exhibited potential
bactericidal activity against seven strains of pathogenic,
gram-negative bacteria as is evident from Table 4.
stereomers, and completely rules out the risk of con-
tamination from the inactive diastereomer, ultimately
reaching the maximal bactericidal and bacteriolytic
effect. However studies are still in progress, to test the
toxicity as well as the mechanism of antibacterial action.
The discovery, modification and annotation of quinoline
diastereomers in terms of their ability to perturb bio-
logical targets will have an important role in the post
genomic era.
Acknowledgements
Encouraged by the above results, the active diastereo-
mer was further screened for its in vitro bacteriolytic
activity to ascertain its potentially active nature. The
results of the bacteriolytic study are summarized in
Table 5. The active diastereomer 7 exhibited potential
bacteriolytic activity against Vibrio parahaemolyticus
and Vibrio vulnificus.
The author gratefully acknowledges the financial sup-
port from the Council of Scientific and Industrial Re-
search, New Delhi, India for this research work. One of
the authors C.J.M. is grateful to Mr. Vijay Krishna,
IIT––Madras, for his timely and valuable help.
References and notes
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In conclusion the paper describes: (1) the discovery of
pyranoquinolines16 as potential antibacterial agents; (2)
The bactericidal and bacteriolytic studies revealed that
one diastereomer was potentially active while the other
was not only inactive but also brought about the com-
petitive inhibition of the active diastereomer 7 by
binding either to the receptor sites on the bacteria or to
some component of the effector mechanism, thereby
preventing the active diastereomer 7 from having an
effect; (3) The first, target-oriented synthesis of pyrano-
quinolines as potential antibacterial agents by inverse
electron demand (IED) Diels–Alder reaction using chi-
ral salen–AlCl complex has been accomplished; (4) The
effect of solvent, the effect of substituents on the salen–
AlCl backbone and the effect of molecular sieves were
probed to synthesis the target pyarnoquinoline as highly
pure diastereomer; (5) The target-oriented synthesis of
the active diastereomer offers the advantages of
enhanced activity, smaller doses, increased specificity,
circumvents the problem of effective separation of dia-

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11. Lucchini, V.; Prato, M.; Scorrano, G.; Tecilla, P. J. Org.
Chem. 1988, 53, 2251–2258.
12. Atwood, D. A.; Jegier, J. A.; Rutherford, D. Inorg. Chem.
1996, 35, 63–70.
suspension in TBS was added to it. The initial OD of the
sample was recorded. The mixture was incubated for
90min at 23 ꢁC. Final OD of the mixture was
recorded. The initial OD)final OD, gives the bacteriolytic
activity.
13. General procedure for the preparation of pyranoquino-
lines: To a mixture of imine (1.1 mmol) or a mixture of
phenylglyoxal monohydrate (1.1 mmol) and aromatic
amine (1.1 mmol), dienophiles [cyclopentadiene (2.2 mmol)
or 3,4-dihydropyran (2.2 mmol) or dihydrofuran
(2.2 mmol)] in dry acetonitrile (5 mL) salen–AlCl complex
(0.22 mmol) was added and stirred at room temperature
for appropriate times. To the reaction mixture water was
added (25 mL) and extracted with CHCl3 (4 · 10mL),
washed with brine and dried over anhydrous Na2SO4,
filtered and the solvent evaporated. The residue was
purified by column chromatography with petroleum ether/
ethyl acetate to afford the cycloadducts.
14. Procedure for bactericidal activity: The test compound
(20mg) was dissolved in 500 lL of DMSO. Five microli-
tres (0.2 mg approx) of the stock solution was taken and
95 lL bacterial suspension in Tris buffer saline (0.8 OD at
580nm) was added to it. The mixture was incubated at
14 ꢁC for 14 h. After incubation, it was subjected to plating
in TCBS agar (Thiosulphate, Citric, Bile salt, sucrose
agar). After 12 h, the culture plate was observed for
bacterial growth.
16. All the compounds were characterized by FT-IR, NMR
(500 MHz), MS and CHN analysis. The spectroscopic
data were fully consistent with the assigned structures.
Selected spectroscopic data: Pyran adduct (6) FT-IR
(KBr): 3380, 3325, 2940, 1603, 1484, 1074 cmÀ1
;
1H
NMR d (500 MHz, CDCl3): 7.47–7.39 (m, 5H), 7.34 (t,
1H, J ¼ 6:8 Hz), 7.1 (t, 1H, J ¼ 8:0Hz), 6.82 (t, 1H,
J ¼ 7:4 Hz), 6.6 (d, 1H, J ¼ 6:8 Hz), 5.35 (d, 1H, J ¼
5:75 Hz), 4.7 (d, 1H, J ¼ 2:2 Hz), 3.9 (br s, 1H, NH), 3.62
(d, 1H, J ¼ 11:4 Hz), 3.4 (m, 1H), 2.21–2.16 (m, 1H), 1.62–
13
1.30(m, 4H);
C NMR d (75 MHz, CDCl3): 145.3,
141.26, 128.5, 128.2, 127.7, 127.6, 126.9, 120.0, 118.4,
114.5, 72.8, 60.7, 59.4, 39.0, 25.5, 18.1; MS m=z 265 (Mþ);
Anal. Calcd for C18H19NO: C, 81.48; H, 7.22; N, 5.28.
Found: C, 81.36; H, 7.21; N, 5.26. Pyran adduct (7) FT-IR
(KBr): 3368, 3026, 2936, 1610, 1489, 1073 cmÀ1; 1H NMR
d (500 MHz, CDCl3): 7.45–7.33 (m, 5H), 7.26 (d, 1H,
J ¼ 6:8 Hz), 7.11 (t, 1H, J ¼ 8:0Hz), 6.73 (t, 1H, J ¼
6:9 Hz), 6.54 (d, 1H, J ¼ 8:0Hz), 4.74 (d, 1H, J ¼
10:85 Hz), 4.42 (d, 1H, J ¼ 2:8 Hz), 4.10(m, 2H), 3.75 (t,
1H, J ¼ 11:45 Hz), 2.11 (m, 1H), 1.88 (m, 1H), 1.71–1.30
(m, 3H); 13C NMR d (75 MHz, CDCl3): 144.8, 142.4,
131.0, 129.4, 128.7, 128.0, 127.9, 120.7, 117.5, 114.2, 74.6,
68.7, 54.9, 39.0, 24.2, 22.1; MS m=z 265 (Mþ); Anal. Calcd
for C18H19NO: C, 81.48; H, 7.22; N, 5.28. Found: C,
81.45; H, 7.23; N, 5.25.
15. Procedure for bacteriolytic activity: Bacterial suspension
in TBS was prepared with an optical density of 0.8 OD at
580nm (double beam UV spectrometer). TBS (Tris buffer
saline) served as the blank. The test compound (10mg)
was dissolved in 150L of DMSO and 2850 lL of bacterial